Conventional contact lenses are generally designed to improve the visual acuity of the human eye. The human eye frequently does not constitute a perfect optical system, but instead suffers from optical imaging errors which are known as ocular aberrations.
Ray tracing is one method of analyzing an optical system. It usually involves calculating parallel light rays entering an optical system and calculating the path of each ray of light. In some instances, the ray trace is included in the lens calculation to control the power profile of the lens.
In the case of soft contact lenses, ray tracing can be used to control the power profile of a lens. This power profile, if held constant, will correct spherical aberration and if properly varied, can correct for presbyopia. Soft contact lenses are usually designed to drape on the eye. The posterior surface of the lens will assume the shape of the eye and the power of the lens is added to the power of the eye. This is true for the entire power profile. The ray trace of the soft lens can be calculated in air since the power profile is added to the eye to cause the desired correction to the eye. The required correction is usually in the form of a Distance Power and an Add Power.
In the case of rigid or semi-rigid contact lenses with a spherical posterior curve, if the posterior curve is selected equal to the curvature of the eye, then the in-air ray trace power profile will add to the power of the eye as does the soft lens. However, if for fitting purposes, the posterior curve needs to be steeper by 0.5 diopters, then the entire power profile of the lens will be decreased by 0.5 diopters in order to maintain the same on-eye powers prior to the fitting change. The in-air ray trace works well for these lenses because the posterior lens curve closely matches the curvature of the eye.
A large number of rigid or semi-rigid contact lenses are designed with aspheric posterior curves that have various eccentricities. For a family of these lenses, where optic zone and posterior eccentricity are held constant, the add power of the lens will change with the posterior central radius such that a smaller conic section will have a larger power shift than a larger conic section. The add power will also change with the power of the lens because of spherical aberration. This will increase the add power on plus power lenses and reduce the add power on negative power lenses.
The posterior asphere can serve to stabilize the fit of the lens on the cornea and to provide a positive power shift from the center of the lens to the edge of the optic zone. The anterior curve of these lenses are usually spherical or, for higher adds, bifocal curves are used. The distance power is usually in the center of the lens and the posterior asphere provides peripheral positive power shift. When an anterior bifocal curve is used, the plus power is out from the center of the lens.
Ray tracing a lens with a posterior asphere in air shows a large power shift due to the posterior aspheric surface. This power shift is caused by the change in radius multiplied by the change in index of refraction from lens material to air. A typical fluoropolymer lens material has an index of refraction of approximately 1.466. Air has an index of refraction of approximately 1.0008. The power shift contribution due to the posterior asphere in air is the change in posterior radius (delta R) multiplied by the change in index of refraction at the back of the lens, i.e. (delta R)*(1.466-1.0008). However, when placed on the eye, the back of the lens is filled with tears having an index of refraction of approximately 1.336 and the calculation for the power shift becomes (delta R)*(1.466-1.336). Therefore, the on-eye power shift is less than 28% of that calculated in air for the same aspheric posterior curve.
Ray tracing can be used to control a power profile that causes a power change as a function of distance from the center of the lens. If the power profile is calculated in air, the profile on the eye will not be the same. The index of refraction in air (1.0008) is different from the tears of the eye and the eye (1.336). Furthermore the normal eye does not show spherical aberration.
There is a need to provide a lens design that incorporates a method of obtaining a lens that exhibits the desired power profile in situ.
However, in view of the prior art taken as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the identified need could be fulfilled.